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AUSABLE WATER WISE: Creating an updated map of Mirror Lake

On a cold rainy day, last May Ausable River Association and Adirondack Watershed Institute staff slipped a boat into Mirror Lake. We were setting out to conduct the first comprehensive survey of the lake bottom. The study of lake and ocean depth is referred to as bathymetry, originating from Greek words for ‘deep’ and ‘measure.’

Henry David Thoreau was one of the first to create a bathymetric map of a lake in the U.S. He used a weight on a rope, lowered through holes drilled in the frozen surface of Walden Pond, to create a map of the pond’s depth. This simple technique of measuring the depth of a lake or pond continues to be used today. New technology was developed in the early 1900s to aid ship navigation, partly driven by the sinking of the Titanic. Acoustic signals, or echo-sounding, allowed ships to detect icebergs and shoals that might be dangerous. This technology is now widely used by ships, anglers, and scientists to understand the depths of oceans, lakes, and rivers. Over the last several decades, the availability, size, and price of echo sounding technology have made it possible to deploy on even the smallest lakes.

Why study the shape and depths of Mirror Lake? Curiosity was likely the inspiration for Thoreau on Walden Pond. We share that same wonder about the natural world that he did but have a few practical reasons as well. Knowing the shape of Mirror Lake helps us understand how salt flows into and accumulates within it. It helps us determine where lake trout and other fish species may seek refuge during the warm summer months and spawn in the fall. And it allows us to calculate the volume of the lake, giving us information on how much salt is trapped within it and how long water resides in the lake.

Thoreau created his map of Walden Pond from a few dozen measurements of its depth. In our mapping of Mirror Lake, we transected the lake every 30 meters, collecting over 34,000 depth soundings. The resulting map shows the lake bottom in greater detail than has ever been seen before. We discovered a low ridge running north-south through the center of the lake and a well-defined second basin on the south end. We captured shallow shoals on the east side, as well as the steep drop along the west. Our map is the equivalent of going from an old blurry photograph to an ultrasharp high definition image.

I am frequently surprised at how little we know about the lakes and ponds that mean so much to us. Most Adirondack lakes haven’t been sampled or surveyed since the Adirondack Lake Survey in the early 1980s. I don’t have enough fingers and toes to count the number of times I’ve relied on an old bathymetric map to sample a lake, only to find the deepest spot isn’t where the map indicates. Good stewardship of our Adirondack waters requires that we know and understand them, and there is so much we have yet to learn.